1. Koni sells shocks called Sports. They are widely known as "yellows" because that's their color... Ground-Control is a completely separate company that sells sleeve coilovers that fit over the Koni shocks. When these 2 are paired together, they make a really great kit that will last forever and out perform many other aftermarket suspension setups.
2. Again, Koni makes the Sport(yellow) shocks. That's what most people use with the GC coilovers. Ground-Control sells their sleeves with Eibach springs which are good. You need to give us some sort of idea of what you want out of your suspension and what you need it to do to determine what spring rate to get.
3. You can get really low with these. HOWEVER, the Koni shocks limit how low you can go because they retain similar dimensions to the stock shocks. So the question should be, can you lower the car enough to scrap the ground or tuck the wheel? Sure. Can you actually drive around like that comfortably and have the suspension actually serve it's purpose, NO. So you can get pretty low, but only to a certain point before you lose suspension travel and you just hit or ride the bump stops.
Again, Koni and Ground-Control are separate companies BUT you can buy this setup together as a package deal(which is usually the cheapest and best way) OR you can buy them separately. Either way they will work.
Ground-Control makes their sleeves to fit different companies shock bodies. So you need to order Koni specific Ground Control sleeves. You CANNOT order a KYB sleeve and expect it to fit on the Koni shocks, because they won't fit.
In your case, the spring rate will mainly be determined by how low you want to go. So tell us that and we can point you in the right direction for spring rates.
Koni/Ground-Control setup is by far the best you can buy for the money. The others you mentioned just don't hold a candle as far as longevity, warranty, performance, pricing, etc.. If you can afford this setup, buy it. If you cannot, then look to others.
BTW, putting a bunch of question marks after words or at the end of sentences won't make us give you a better answer. It just makes you look like you're trying to make this purchase on a whim and tells us you're not doing enough research yourself.